Honestly I just put several apps into sleep or deepsleep and when I really need battery I turn on DND.
I don't know why the changes are so drastic but in my device battery went up from 3-4hrs SOT to 6-7hrs SOT and from 12-16hrs stand by to 24-30hrs stand by.
Also just to clarify, I live in Brazil so there is no 5G here, only 4G. I also deactivate RAM Plus.
I do have to admit though, the default S21 FE experience leaves much to be desired because the battery is not enough.